Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Applications for fuel cells include battery replacement, mini and microelectronics, car engines, power plants, and many others. One advantage of fuel cells is that they are substantially pollution-free.
In hydrogen fuel cells, hydrogen gas is oxidized to form water, with a useful electrical current produced as a byproduct of the oxidation reaction. A solid polymer membrane electrolyte layer may be used to separate the hydrogen fuel from the oxygen. The anode and cathode are arranged on opposite faces of the membrane. Electron flow between the anode and cathode layers of the membrane electrode assembly may be exploited to provide electrical power. Hydrogen fuel cells are impractical for many applications, however, because of difficulties related to storing and handling hydrogen gas.
Organic fuel cells may prove useful in many applications as an alternative to hydrogen fuel cells. In an organic fuel cell, an organic fuel such as methanol is oxidized to carbon dioxide at an anode, while air or oxygen is simultaneously reduced to water at a cathode. One advantage over hydrogen fuel cells is that organic/air fuel cells may be operated with a liquid organic fuel. This eliminates problems associated with hydrogen gas handling and storage. Some organic fuel cells require initial conversion of the organic fuel to hydrogen gas by a reformer. These are referred to as “indirect” fuel cells. The need for a reformer increases cell size, cost, complexity, and start up time. Other types of organic fuel cells, called “direct,” eliminate these disadvantages by directly oxidizing the organic fuel without conversion to hydrogen gas. To date direct organic fuel cell development has focused on the use of methanol and other alcohols as fuel.
Conventional direct methanol fuel cells have unresolved problems associated with them. For example, methanol and other alcohols have high osmotic and diffusion crossover rates across commercial polymer membrane electrode assemblies. Fuel that crosses over avoids reaction at the anode, and thus cannot be exploited for electrical energy. This limits cell efficiency. An additional problem related to crossover is poisoning of the anode. As methanol or another alcohol fuel crosses over the polymer membrane to the cathode side, it adsorbs onto the cathode catalyst and thereby blocks reaction sites. Efficiency of the cell is thereby reduced. A proposed solution to this problem has been to provide additional catalyst. This adds expense, however, particularly when considering that costly precious and semi-precious metal catalysts such as platinum are often employed.
Because of this high crossover, methanol and other alcohol fuel cells typically operate with a fuel concentration of no more than about 3-8%. The use of those dilute solutions creates additional problems, however. This low fuel concentration requires relatively large amounts of ultra-pure water, typically provided through recycling systems including pumps and filters. Also, the concentration of the fuel needs to be closely monitored and controlled, with the result that sensors and controllers may be required. All of this peripheral equipment adds cost, complexity, weight, and size to direct organic fuel cells.
In addition, this required peripheral water management equipment substantially limits the usefulness of direct methanol fuel cells for applications where size and weight become critical. For portable, miniature, and microelectronics applications, for example, the size, weight, and complexity of the required peripheral equipment makes use of direct methanol fuel cells impractical.
Further, the dilute solutions freeze and expand at temperatures potentially encountered in many fuel cell applications, with portable devices for use outside as an example. The expansion can lead to device failure. Conduit et al. U.S. Pat. No. 6,528,194 teaches that the freezing can be avoided by circulating heated fluid through the fuel tank when the fuel cell is not operating. However, that wastes power and adds complexity.
Still other problems with existing direct methanol fuel cells relate to the electro-oxidation reaction promoted by the anode. For example, an intermediate produced during the oxidation/reduction reaction from the methanol in many direct methanol fuel cells is poisonous carbon monoxide gas. Thus hazards are presented. Also, CO is known to poison catalysts such as Pt and to thereby decrease cell efficiency.
These and other problems remain unresolved in the art.